Hybrid voltage/current-mode transmission line driver

Abstract

A system that transmits signals through a communication channel. During operation, the system receives a sequence of bits for transmission through the communication channel. While transmitting a given bit, the system determines if the given bit has the same state as the previously transmitted bit. If so, the system uses a voltage-mode driver to drive a signal through the communication channel. Otherwise, the system uses a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver. Note that the current source supplies a current to the communication channel without changing the impedance of the voltage-mode driver. In this way, the present invention compensates for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption.

1. A method for transmitting signals through a communication channel, comprising:
 receiving a sequence of bits for transmission through the communication channel; 
 determining if a given bit in the sequence of bits has the same state as the previously transmitted bit; 
 if so, using a voltage-mode driver to drive a signal for the given bit through the communication channel; and 
 otherwise, using a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit; 
 wherein a value of a resistor couple for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver, substantially equals the impedance of the communication channel and wherein the current source supplies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption. 
 
     
     
       2. The method of  claim 1 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
     
     
       3. The method of  claim 1 ,
 wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and 
 wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
 
     
     
       4. The method of  claim 1 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel. 
     
     
       5. The method of  claim 1 , wherein the voltage-mode driver is a differential driver. 
     
     
       6. An apparatus for transmitting signals through a communication channel, comprising:
 a communication channel; and 
 a transmitter; 
 wherein the transmitter is configured to:
 receive a sequence of bits for transmission through the communication channel; 
 determine if a given bit in the sequence of bits has the same state as the previously transmitted bit; 
 if so, to use a voltage-mode driver to drive a signal for the given bit through the communication channel; and 
 otherwise, to use a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit; 
 wherein a value of a resistor coupled for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver substantially equals the impedance of the communication channel and wherein the current source applies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption. 
 
 
     
     
       7. The apparatus of  claim 6 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
     
     
       8. The apparatus of  claim 6 ,
 wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and 
 wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
 
     
     
       9. The apparatus of  claim 6 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel. 
     
     
       10. The apparatus of  claim 6 , wherein the voltage-mode driver is a differential driver. 
     
     
       11. A computer system for transmitting signals through a communication channel, comprising:
 a communication channel; and 
 a transmitter; 
 wherein the transmitter is configured to:
 receive a sequence of bits for transmission through the communication channel; 
 determine if a given bit in a sequence of bits has the same state as the previously transmitted bit; 
 if so, to use a voltage-mode driver to drive a signal for the given Bit through the communication channel; and 
 otherwise, to use a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit; 
 wherein a value of a resistor coupled for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver substantially equals the impedance of the communication channel and wherein the current source applies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption. 
 
 
     
     
       12. The computer system of  claim 11 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
     
     
       13. The computer system of  claim 11 ,
 wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and 
 wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal. 
 
     
     
       14. The computer system of  claim 11 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel. 
     
     
       15. The computer system of  claim 11 , wherein the voltage-mode driver is a differential driver.